1. Field of the Invention
The present invention relates to an ignition control system for internal combustion engines and more particularly to an ignition coil ignition control system which is useful in case of a failure of a computing ignition control unit or the like. The internal combustion engine will hereinafter be simply referred to as an engine.
2. Description of the Prior Art
With a view to meeting the requirements of the reduced ignition coil size and the increased rotational speed, there has recently been a tendency toward increasing the flow of energizing current to the ignition coil and decreasing the dwell period. Techniques are known in the art by which the duration of current flow to the ignition coil is controlled at a constant duration time in accordance with the battery voltage by means of a computing control unit using a microcomputer and the control units of this type are shown, for example, in Japanese Laid-Open patent applications Nos. 55-5406 and 55-54669.
However, the computing control units of the type employing a microcomputer for controlling the dwell period must ensure the function of interrupting the current flow to the ignition coil when the microcomputer fails. Therefore, while it is conceivable to substitute a timer circuit for the back-up function which ensures the function of interrupting the current flow to the ignition coil, there is a disadvantage that in order to overcome the danger of a power transistor being broken by an excessively large current or the ignition performance being deteriorated by an excessively small current, the timer circuit must be provided with an additional function for varying its control of maintaining constant the duration of current flow to the ignition coil in accordance with the battery voltage and otherwise the timer circuit cannot be useful as a protective circuit capable of performing the back-up function.